It is well known that humidity is important to comfort within any given environment. Many systems have been developed to adjust humidity levels in an enclosure, either by adding moisture to the air or removing moisture for given temperature conditions. The majority of prior art systems are involved with controlling the ‘relative humidity’ as opposed to the actual ‘humidity’, even though the literature relating thereto is generally expressed simply in terms of ‘humidity’.
The term ‘absolute humidity’ is an expression of the actual amount of water vapor present in the air, and is usually expressed in terms of a humidity ratio, or pounds of water vapor per pound of dry air. ‘Dewpoint’ and ‘absolute humidity’ are related in that they both relate to the total amount of water in the air. As is known, ‘dewpoint’ is the temperature at which air become totally saturated, such that moisture condenses. For each value of ‘absolute humidity’ there is a single related value of ‘dewpoint’. Unless water is added or taken away, the ‘absolute humidity’ and the ‘dewpoint’ remain the same, even though the temperature of the air changes.
The amount of moisture that is in the air, when compared to the maximum amount of moisture it can hold at a given temperature, is referred to as ‘relative humidity’. As noted, this is the relationship that is generally cited as ‘humidity’ when literature or people refer to humidity-related numbers. Unless otherwise qualified, the term ‘humidity’ will be utilized in hereinafter in the general usage sense to indicate relative humidity.
It is of course well-known that warm air can hold more moisture than can cold air, and that the ‘relative humidity’ changes with changes in temperature, even though no moisture is added or removed. If a humidity control system maintains a constant humidity as temperature rises, a substantial amount of additional moisture will be held in the air. Though it would be desirable to maintain dewpoint at around 50° from a comfort standpoint, cold outside temperature may make this undesirable due formation of condensation on windows or wall structures. Problems can occur when comfortable indoor humidity levels are maintained during cold weather. Moisture will usually first condense on windows because they have less insulation capability than walls, that is, the temperature on the inside surface may be lower than the dewpoint of the inside air.
The American Association of Heating and Refrigeration Engineers recommends maintaining humidity levels for indoor swimming pools and spas in the range of 50% to 60% to provide the most desirable for comfort. It also recommends that the building enclosure be designed to handle these levels to avoid damage from condensation, frost build-up, and moisture penetration. In geographical areas with cold winter climates, this ideal cannot always be met. In some buildings and conditions, it is necessary or desirable to adjust the humidity levels as the outside temperature changes so as to avoid or minimize damage to the structure.
Some manufacturers have recognized and provided equipment that addresses the humidity concerns in residential structures. For example, one supplier has marketed a system for residential humidification, but not dehumidification, as a function of outside temperature. However, the ranges of humidity control for that system are too low for swimming pool or spa applications, and merely function as on-off control, not proportional control. Another manufacturer has marketed a residential system that calculates and adjusts dewpoint, but again, the ranges of control are insufficient for use in swimming pool applications.
While simply reducing the humidity within the building can have a positive result in lowering the damages mentioned above, there is cost factor in such reduction. For example, reducing the humidity in a pool or spa structure causes the rate of evaporation to increase. This increases the cost to remove the additional water vapor. For example, in a typical indoor pool, reducing the indoor humidity for 55% to 35% increases the evaporation rate by about 30%. Energy costs to control this higher level of humidity reduction (removal of additional moisture) will usually increase by a similar rate. Prior art humidity control systems adapted for use in pool or spa enclosures have been of fixed setting variety that either maintain humidity at fixed levels based upon inside temperature, or require manual re-setting to account for temperature changes. Such manual intervention results in inefficient energy usage and is difficult to maintain on regular basis to account for temperature changes from daytime to nighttime conditions.
Rather than maintaining humidity at a fixed rate, with the attendant additional costs, or attempting to manually adjust humidity targets, it would be preferable to adjust the inside humidity change as a function of the inside humidity and the outside temperature.